There are several methods for removing, or stripping, materials such as organic photoresist or other organic residues, collectively referred to as "photoresist", from semiconductor wafers. One common method is the use of a heated hydrogen peroxide and sulfuric acid mixture. The mixture of hydrogen peroxide and sulfuric acid in a processing bath produces a volatile exothermic reaction which removes the photoresist by oxidizing the carbon in the photoresist. The carbon is liberated from the photoresist in the form of carbon dioxide gas. Some of the hydrogen peroxide is also converted to water which, depending on the bath condition, remains in the bath or boils out. The rate at which hydrogen peroxide is exhausted from the bath is dependent on the temperature, photoresist content, and acid concentration.
It is important to maintain the correct mixture of hydrogen peroxide and sulfuric acid so that consistent photoresist stripping is obtained. A traditional method for insuring consistent hydrogen peroxide to sulfuric acid mixture is to observe the processing bath for bubbles. Vigorous bubbling indicates an adequate amount of hydrogen peroxide to remove, or strip, the photoresist. Over time and usage, the hydrogen peroxide concentration will diminish so constant operator attention to the bath is needed to ensure consistent bubble activity.
Most modern processing baths are designed with covers so a visual inspection of the bath condition is not easily done. These newer systems usually add hydrogen peroxide to the bath on a time or wafer batch basis which can be costly or inadequate. If the bath is idle and chemicals are being added on a time basis, expensive chemicals are being wasted. Also the bath could become too diluted with water. Wafer batch addition of hydrogen peroxide can be inadequate because the hydrogen peroxide concentration decreases over time requiring more hydrogen peroxide as time increases between batches. If the time between batches is sporadic, the amount of hydrogen peroxide is difficult to determine. Both time and batch addition of hydrogen peroxide also fail to compensate for varying amounts of photoresist on the wafers. More photoresist on the wafers will consume more hydrogen peroxide.
Analytical testing systems which measure the proportion of hydrogen peroxide to sulfuric acid are expensive. Further, most of these systems require the mixture to be at around room temperature so they can not be used as a part of a real-time control system. A simple system is required which can monitor the bath and feed back of this information so that a consistent mixture can be maintained throughout the wafer processing.
A solution for solving this problem simply and inexpensively has been long sought by and eluded those skilled in the art.